(1) Field of the Invention
The present invention relates to semiconductor devices and methods for fabricating the same, and more particularly relates to semiconductor devices offering enhanced speed and enhanced degree of integration and methods for fabricating the same.
(2) Description of Related Art
In recent years, with miniaturization in semiconductor process rules, influences exerted by variations in the geometries of large scale integration (LSI) patterns and a deviation from precise alignment of a mask in formation of the LSI patterns have become apparent. This hampers reduction in the area of a semiconductor device. This will be initially described with reference to FIGS. 3A and 3B.
FIG. 3A is a plan view illustrating an exemplary metal oxide semiconductor (MOS) transistor 10 in a known semiconductor device. To be specific, FIG. 3A illustrates patterns of an impurity region 11 formed on a substrate (not shown), gate electrodes 12 partly formed on the impurity region 11, and gate contacts 13 formed on respective parts of the gate electrodes 12 to provide electrical connection with the gate electrodes 12. Although not shown, interconnects are formed on the gate contacts 13.
The patterning accuracy of fine LSI patterns has a ceiling. Therefore, for example, a deviation from the designed value is caused due to variations in the shape of each of the finished gate electrodes 12. Furthermore, a deviation from the designed location of each gate contact 13 relative to associated one of the gate electrodes 12 is to be formed is caused due to a deviation from precise alignment of a mask. To cope with the above, each gate electrode 12 is formed as a pattern having such a shape that includes a land part 12a and a gate body part 12b to compensate for the deviations from the design value and the precise pattern alignment due to the lack of the patterning accuracy of the LSI patterns and provide a margin that ensures electrical connection between the gate electrode 12 and associated one of the gate contacts 13. The size (area) of the land part 12a is large enough to ensure connection with the gate contact 13. Furthermore, the land part 12a is formed apart from the impurity region 11 to prevent an influence from being exerted upon the performance of the MOS transistor 10. A plurality of gate body parts 12b may be connected to a single land part 12a. 
In the MOS transistor 10, the distance S1 between the impurity region 11 and each land part 12a is also set to have a margin for compensating for a deviation between the actually finished shape of the MOS transistor 10 and the designed shape thereof due to the lack of the patterning accuracy. Furthermore, the margin S2 between the land part 12a and associated one of the gate contacts 13 is also set large enough to compensate for a deviation from design due to the lack of the patterning accuracy.
A deviation from the designed value due to the lack of the patterning accuracy, the distance S1 and the margin S2 will be described hereinafter with reference to FIG. 3B.
FIG. 3B illustrates a state in which in formation of MOS transistors 10 including an impurity region 11, gate electrodes 12 and gate contacts 13 as illustrated in FIG. 3A, a deviation from design due to the lack of the patterning accuracy is very noticeably caused.
First, parts of the gate electrodes 12 corresponding to the corners thereof when viewed in plan are rounded by the phenomenon called rounding so as to be formed into shapes in which the outlines of the gate electrodes 12 are deviated from the designed outlines thereof (finished shapes 22). More particularly, the finished shapes 22 are as follows: Outwardly extending ones of the corners of the gate electrodes 12 are rounded to have outlines located inside the outline of the designed patterns when viewed in plan. On the other hand, inwardly extending ones of the corners of the gate electrodes 12 are rounded to outwardly project beyond the designed patterns when viewed in plan. As a result, part of each gate body part 12b located in the vicinity of the land part 12a has a longer gate length than the other part thereof.
Such a rounding phenomenon is remarkably caused, for example, in cases where a pattern to be formed has a complicated shape and where the density of patterns significantly varies across the substrate surface.
In FIG. 3B, a deviation S3 from precise alignment of each gate electrodes 12a relative to the impurity region 11 (hereinafter, referred to as “alignment deviation S3”) causes that the distance S4 between the impurity region 11 and each land part 12a after finishing becomes smaller than the distance S1 in FIG. 3A.
In a case where the distance S4 after finishing becomes too small, this causes that part of each gate body part 12b having a longer gate length than the other part thereof due to rounding is located on the impurity region 11. In this case, the MOS transistor 10 has a different gate length from a designed gate length. This affects the performance of the so-constructed MOS transistor. The MOS transistor 10 is designed under consideration of the finished shape 22 and the alignment deviation S3. This prevents part of each gate electrode 12 extending beyond the designed outline of the gate electrode 12 due to rounding of the finished shape 22 from overlapping with the impurity region 11 even when the above-mentioned effect is produced to the maximum extent possible. This is achieved by setting the designed distance S1 between the impurity region 11 and the land part 12a in FIG. 3A large enough (to provide a margin for compensating for deviations).
In FIG. 3B, a deviation from the designed location of each gate contact 13 relative to the associated land part 12a (for example, a deviation S5 from the designed location of the gate contact 13 (hereinafter, referred to as “contact location deviation S5”)) and any other deviation are also caused. When the corners of the land part 12a are rounded due to rounding to a large extent and the contact location deviation S5 becomes large, there is a possibility that the gate contact 13 could not remain within the land part 12a (the contact 13 may be only partly formed on the land part 12a). This causes variations in resistance and disconnection. To cope with this, the margin S2 illustrated in FIG. 3A is set large enough to prevent the gate contact 13 from being only partly formed on the land part 12a even when the influence of the deviation between the finished shape 22 of the gate electrode 12 and the designed shape thereof due to rounding is produced to the maximum extent (in other words, the margin S2 is set to be large enough).
Next, FIGS. 4A and 4B illustrate an example of a technique for reducing the margin for alignment between a gate electrode and a contact providing connection with an impurity diffusion region in a known semiconductor device (hereinafter, referred to as “impurity diffusion region contact”).
FIG. 4A is a plan view illustrating a known semiconductor device to which a technique for forming a self-aligned contact hole has been applied. The semiconductor device includes impurity regions 41 formed in the upper portion of a substrate (not shown) and forming source and drain regions, gate electrodes 42 formed on the impurity regions 41, and an impurity region contact 43 for providing electrical connection with one of the impurity regions 41. FIG. 4B is a cross-sectional view taken along the line IVb-IVb′ in FIG. 4A.
In FIGS. 4A and 4B, each of gate electrodes 42 is formed on a substrate 40 with associated one of gate insulating films 44 interposed therebetween, and impurity regions 41 are formed in parts of the substrate 40 located to both sides of the gate electrode 42. A protective insulating film 47 is formed on the gate electrode 42, and sidewalls 46 are formed on both sides of a set of the gate electrode 42 and the protective insulating film 47. The gate electrode 42 and some other elements are covered with an interlayer insulating film 45. An opening is formed to pass through the interlayer insulating film 45, and an impurity region contact 43 for providing connection with the impurity region 41 is formed to fill the opening.
With the above-mentioned structure, even when the location at which the impurity region contact 43 is formed relative to the gate electrode 42 is deviated from the designed location, the protective insulating film 47 formed on the gate electrode 42 prevents electrical shorting between the gate electrode 42 and the impurity region contact 43. Therefore, margins for variations in the accuracy of the alignment between the gate electrode 42 and the impurity region contact 43 and variations in the accuracy of the size of the finished gate electrode 42 and the finished impurity region contact 43 do not need to be provided. As a result, the distance between the gate electrode 42 and the impurity region contact 43 is reduced, thereby increasing the degree of integration of an LSI. This technique is disclosed, for example, in Japanese Patent Publication No. 2666325 (in particular, page 4, FIG. 1).
FIG. 5 illustrates the state of a known semiconductor device in which lower interconnects 51 (51a, 51b, 51c, and 51d) for element-to-element connection (hereinafter, referred to as “element-to-element connection lower interconnects 51”) and upper interconnects 53 (53a and 53b) for element-to-element connection (hereinafter, referred to as “element-to-element connection upper interconnects 53”) located above the element-to-element connection lower interconnects 51 are provided and the element-to-element connection lower interconnects 51 are connected through vias 52 (52a and 52b) to the element-to-element connection upper interconnects 53. More particularly, a via 52a is formed to come into contact with the top surface of a part of an element-to-element connection lower interconnect 51b, and further an element-to-element connection upper interconnect 53a is formed to come into contact with the top surface of the via 52a. In this way, the element-to-element connection lower interconnect 51b is electrically connected to the element-to-element connection upper interconnect 53a. Likewise, an element-to-element connection lower interconnect 51d is electrically connected through a via 52b to an element-to-element connection upper interconnect 53b. 
Also when the vias 52 are formed to achieve the above-mentioned structure, the locations at which the vias 52 are actually formed may deviate from predetermined locations at which the vias 52 are to be formed. As a result, the element-to-element connection lower interconnects 51 are at risk of being prevented from ensuring electrical connection with the vias 52. To avoid this, it should be considered that as in the case of the gate electrodes illustrated in FIG. 3A, parts of the element-to-element connection lower interconnects 51 are enlarged so as to be formed into land parts. However, the distance S50 between any adjacent two of the element-to-element connection lower interconnects 51 is specified as the patternable minimum interconnect-to-interconnect distance. Therefore, the width of each element-to-element connection lower interconnect 51 cannot be set arbitrarily. In view of the above, the element-to-element connection lower interconnect 51 cannot be arbitrarily provided with a land part.
To cope with this, for example, as illustrated in FIG. 5, the via 52a is formed a predetermined distance S51, i.e., a margin, behind one end of the element-to-element connection lower interconnect 51b. In other words, the element-to-element connection lower interconnect 51b is formed to outwardly project the distance S51 beyond the via 52a. When the distance S51 is thus long enough, this can prevent the via 52a from being apart from the top surface of the element-to-element connection lower interconnect 51b in the longitudinal direction thereof even with a deviation from the designed location of the via 52a. The distance S52 between the element-to-element connection lower interconnects 51b and 51d is specified as the patternable minimum interconnect-to-interconnect distance.
For this structure, no margin is provided to compensate for deviations from the designed locations of the vias 52a in the direction orthogonal to the longitudinal direction of the element-to-element connection lower interconnect 51b. However, since a margin is provided along at least one direction (the longitudinal direction of the element-to-element connection lower interconnect 51b), the known semiconductor device is designed to ensure electrical connection between the element-to-element connection lower interconnect 51b and the via 52a. 